PVA hydrogel

ABSTRACT

The present invention provides methods of making covalently crosslinked vinyl polymer hydrogels having advantageous physical properties, and covalently crosslinked vinyl polymer hydrogel compositions made by such methods, as well as articles of manufacture comprising such covalently crosslinked vinyl polymer hydrogel compositions. The physical properties of the produced hydrogels can be adjusted by varying controlled parameters such as the proportion of physical associations, the concentration of polymer and the amount of radiation applied. Such covalently crosslinked vinyl polymer hydrogels can be made translucent, preferably transparent, or opaque depending on the processing conditions. The stability of the physical properties of the produced vinyl polymer hydrogel can be enhanced by controlling the amount of covalent crosslinks.

BACKGROUND OF THE INVENTION

Vinyl polymers are used in a variety of industrial applications. Forexample, poly vinyl alcohol (PVA) is a highly hydrophilic polymer thatis used as sizing in the textile industry, as a base gel component forthe cosmetics industry, as an adherent for the paper industry and as ageneral adhesive. The chemical formula of PVA is (C₂H₄O)_(n) and thestructural formula is (—CH₂CH(OH)—)_(n). It is widely known that PVAelicits little or no host biological response when implanted in animals.For this reason PVA is also used in a variety of biomedical applicationsincluding drug delivery, cell encapsulation, artificial tears, contactlenses, and more recently as nerve cuffs. PVA has generally not beenconsidered for use as a load bearing biomaterial, primarily because ofits low modulus and poor wear characteristics. It has been reported inthe literature that hydrogel modulus and wear characteristics can oftenbe enhanced by the formation of either chemical or physicalassociations. Cross-linking PVA by the addition of chemical agents suchas polyaldehydes, through irradiation, or by freeze-thaw cycling, hasbeen shown to improve the durability of PVA.

Use of PVA prepared by freeze-thawing methods has been suggested for usein biomedical applications as early as 1973. U.S. Pat. No. 3,875,302issued to Taisei Inoue on Apr. 1, 1975 described a process of preparinggelled vinyl alcohol polymers by freezing an aqueous solution of a vinylalcohol polymer below about −5 degrees Celsius and thereafter meltingthe frozen solution. The process of forming cryogels by freeze-thawcycling was also described in a 1975 chemical engineering Ph.D. thesisby N. A. Peppas at the Massachusetts Institute of Technology (Cambridge,Mass.) and in U.S. Pat. No. 5,891,826. See also U.S. Pat. Nos.4,472,542, 5,288,503 and 5,981,826; the entire contents of all citedreferences, patents and patent publications are incorporated byreference herein. Because of the slow dehydration exhibited by cryogels,they have been considered for use in contact lenses. PVA has also beenconsidered for drug release applications, especially since thefreeze-thaw process does not affect protein structure. Bioadhesive PVAgels have also been considered.

It is known that the exposure of aqueous solutions of PVA polymer toionizing radiation can produce gels (Wang, B., et al. The Influence ofPolymerconcentration on the Radiation-chemical Yield of IntermolecularCrosslinking of Poly(Vinyl Alcohol) by g-rays in Deoxygenated AqueousSolution. Radiation Physics and Chemistry, 2000. 59: p. 91-95).Irradiation of PVA results in a chemical crosslinking of the polymerchains by the formation of covalent bonds. Hydrogels may be formed byirradiation of solid PVA polymer, PVA monomer (in bulk or in solution)or PVA polymer in solution. Irradiating a hydrophilic polymer in dryform is problematic for a variety of reasons, including the formation ofunstable bonds and oxygen that cannot be fully removed. Additionally,the restricted motion of the polymer chains that bear the reactive freeradicals limits the effectiveness of the cross-linking. In somehydrogels, it is possible to generate a cross-linked polymer solution bystarting with pure monomer. Polymerization is performed first, followedby cross-linking, which is very convenient for many polymers. However,because of the instability of the PVA monomer, this is not a viableapproach for making a PVA hydrogel. For most applications, crosslinkingis conducted on polymer chains that are in solution, preferably in adeoxygenated solution.

To test the biocompatibility of PVA cryogels, Oka et al., implanted thePVA into rabbit patellar grooves and demonstrated little or no hosttissue response. In further experimentation, small particles of the PVAhydrogel or UHMWPE controls, 50-300 microns in diameter, were implantedinto the knee joints of wister rats. The UHMWPE induced a severe tissueresponse while the PVA did not induce a measurable response. The PVA wasalso bonded to a titanium fiber mesh which promoted bony in-growth wheninset into the patellar grove of the femoral heads in rabbits. Thus, thecombination titanium fiber mesh/PVA implant integrated into the jointand provided a reasonable bearing surface for joint loads.

The PVA had a low frictional coefficient when opposing articularcartilage (<0.1). Thus, it is likely that this biomaterial may be usefulin hemi-arthroplasty (where wear against a hard surface is not anissue). To test the biocompatibility in this application, PVA backedwith a titanium mesh was placed into the load bearing region of dogfemoral condyles. The material was tolerated well and induced bonyin-growth for fixation. The conclusion of Oka et al. is that thiscomposite osteochondral device (COD), is ready for more extensiveinvestigation as a partial articular surface replacement device.

The efficiency of a dose of gamma -radiation for crosslinking PVA indeoxygenated aqueous solution as a function of polymer concentration isshown in FIG. 1. Note that there is not only an ideal dose level, butalso an optimal concentration of polymer where the efficiency ofcrosslinking is maximized (˜30-300 g/dm³). The peak in the crosslinkingefficiency at approximately 300 g/dm³ is due to increasing degradation(random scission) of the polymer chains at higher radiation doses.

The relationship between cross-linking and degradation can be understoodby considering the case of irradiated solid PVA. The irradiation ofsolid PVA leads to main chain degradation as a result of ketonestructure formation which is not due to an oxidation step via oxygen,but through keto-enol tautomerization. In keto-enol tautomerism, asimultaneous shift of electrons and a hydrogen atom occurs. Main chainscission can then occur in the backbone bearing the keto tautomer.Keto-enol degradation is thought to dominate when the concentration ofthe polymer limits chain movement and free radical mobility. Thus, asthe concentration passes 300 g/dm³, scission becomes more prevalent.

When ionizing radiation is applied to polymer chains in solution,reactive intermediates can be formed either by direct ionization, orindirectly by interaction with reactive intermediates (hydroxylradicals) in the aqueous solution. In dilute solution, the indirectroute dominates because of the electron fraction of the solution. Thus,for polymers in solution, the indirect route will be the primarymechanism responsible for the formation of reactive intermediates andsubsequently, for the generation of crosslinks or scission. Becausesimple gel forming hydrophilic polymers do not have functional groupscapable of efficient scavenging of free electrons, they do notparticipate in the formation of crosslinks extensively. The realworkhorse is the hydroxyl radical in the aqueous solution. Nitrousoxide, which converts the free electrons to hydroxyl radicals, issometimes added to polymer solutions undergoing radiation inducedcrosslinking to improve yield. Rosiak & Ulanski showed that thedependence of gelation dose (determined by rheology) on concentrationwas found to have a local minimum in the neighborhood of about 20 g/dm³(FIG. 2, from Rosiak, J. M. & Ulanski, P., Synthesis of hydrogels byirradiation of polymers in aqueous solution, Radiation Physics andChemistry 1999 55: 139-151). The method of crosslinking can by optimizedby determining the local minimum in a corresponding gelation dose versusconcentration curve for a given vinyl polymer and performingcrosslinking in that range of irradiation doses.

In deoxygenated solutions, when chain break precursors arecarbon-centered radicals localized at the main chain, the chain scissionreactions are very slow because re-combination of radicals prevails. Fornon-ionic polymers like PVA, under normal irradiation conditions, chainscission yield is near zero if the concentration of polymer is lowenough.

Additives can be used during the irradiation process to scavengeunwanted transient products (for example, tertbutanol scavenges OH— andnitrous oxide scavenges aqueous electrons). Other additives can helpidentify transient reaction products (tetranitromethane helps identifypolymer radicals). Spin traps (2-methyl-2-nitrosopropane) allow EPR (orESR) studies on short-lived species. Thiols are good H⁺ donors and arefrequently used as polymer radical scavengers. Metal ions such as Fe(II)are also known to significantly affect the kinetics and yields ofradiation-induced transformations of polyacrylic acid (PAA) (forexample). Accordingly, all glassware should be carefully cleaned andeven treated with complexing agents such as EDTA to remove traces ofmetal ions when working with polyelectrolyte gels. However, metalcontamination should not cause problems when working with PVA.

Oxygen should also be considered an additive that must be carefullycontrolled. In oxygenated solutions, carbon centered macroradicals reactwith oxygen to form peroxyl radicals. The kinetics of this reaction arequite rapid (practically diffusion limited at a reaction constant of 10⁹dm³/mol/sec). Even in a polyanionic gel, where oxygen approach ishindered by charge effects, the reaction constant is as high as 10⁸dm³/mol/sec. When crosslinking with oxygen present it is important tonote that neither the peroxyl nor the oxyl radicals form stable bondsupon recombination. Additionally, one of the main reaction pathwaysleads to chain scission (see Scheme 1 below). One method is to performthe irradiation in a sealed vessel. The oxygen present will be used upand gelation will occur. Sealed vessel irradiation has been utilized toproduce hydrogel dressings. One could also irradiate in an open vesseland count on the diffusion limitation to slow the transport of oxygenfrom the surface. In this case, a high irradiation dose rate would beadvantageous. It is also possible that a natural oxygen scavenger suchas vitamin E would allow irradiation in an oxygen environment whileminimizing chain scission.

Physical Properties of Irradiated Poly(Vinyl Alcohol) Hydrogels

Irradiated PVA films (⁶⁰Co gamma ray source, nitrogen atmosphere,dose-rate 0.0989 kGy/min, 86 kGy total dose; 10-15 wt % 78 kDa PVA indeionized water) had a tensile strength of 19.7 MPa and a strain of 609%on breaking. Compressive modulus obtained by dynamic mechanical analysis(DMA) on 10% solutions of PVA directly irradiated by electron beam inair (100 kGy total dose) yielded a 0.5 MPa storage modulus at 1 Hz.However, the samples were quite brittle.

Cross-Linking: Freeze-Thaw Cycling

Freeze/thaw cycling of PVA polymer in solution results in the formationof physical cross-linking (i.e. weak bonding through a nonpermanent“association” of the polymer chains). PVA hydrogels formed in thismanner are thermoreversible and are termed “cryogels”. In general,cryogels are solid elastomers containing over 80% water which areproduced when solutions of higher molecular weight poly(vinyl alcohol)(PVA) of high degree of hydrolysis are subjected to one or morefreeze-thaw cycles. Such cryogels are tough, slippery, elastomeric,resilient, insoluble in water below 50 degrees Celsius, and nontoxic.

Freeze-thaw cycling of solutions of PVA polymer results in the formationof physical associations (i.e. weak bonding through an “association” ofthe polymer chains). PVA hydrogels formed in this manner are termed“cryogels” and are described, for example, in U.S. Pat. Nos. 6,231,605and 6,268,405, the entire contents of which are incorporated herein byreference. Importantly, the techniques utilized to create PVA cryogelsdo not require the introduction of chemical crosslinking agents orradiation. Cryogels are therefore easily produced with low impact onincorporated bioactive molecules. However, incorporated molecules arelimited to those that can tolerate the freeze-thaw cycles required tomake the gel. Thus the resulting material can contain bioactivecomponents that will function separately following implantation. PVAcryogels are also highly biocompatible (as are PVA “thetagels,”discussed below). They exhibit very low toxicity (at least partially dueto their low surface energy), contain few impurities and their watercontent can be made commensurate to that of tissue at 80 to 90 wt %.

There is still some debate over the exact mechanism that drives thegelation of PVA through a freeze-thaw cycle. However, three models havebeen proposed to explain the physical crosslinking that occurs duringthe freeze-thaw cycle: 1) direct hydrogen bonding; 2) direct crystalliteformation; and 3) liquid-liquid phase separation followed by a gelationmechanism. The first two steps suggest that the gel forms through anucleation and growth (NG) phase separation, whereas the third optionpictures the process as a spinodal decomposition (SD) phase separation.Hydrogen bonding will form nodes and crystallite formation will formlarger polymer crystals. However both of these mechanisms will formclosely connected crosslinks, with relatively small crosslinking nodes.This observation is supported by studies on the gelation mechanism ofPVA. Spinodal decomposition on the other hand causes redistribution ofthe polymer into polymer rich and polymer poor regions followed by agelation process which results in more distantly spaced crosslinks. Itis thought that phase separation through spinodal decomposition islikely to be responsible for the improved mechanical properties of PVAafter crosslinking and occurs due to a quenching of the polymersolution. During the freezing process, the system undergoes a spinodaldecomposition whereby polymer rich and poor phases appear spontaneouslyin the homogeneous solution. This process occurs because the phasediagram of quenched PVA (and polymers in general) at certaintemperatures can have two coexisting concentration phases. The polymerrich phases are therefore highly concentrated which enhances the natural(weak) gelation of the PVA.

For cryogels, the physical characteristics depend on the molecularweight of the uncrosslinked polymer, the concentration of the aqueoussolution, temperature and time of freezing and the number of freeze-thawcycles. Thus the properties of a cryogel can be modulated. However,since the material's properties change dramatically at every freeze-thawstep, control over the properties of the finished gel is somewhatlimited. The thetagels described broaden the range of functionalitycurrently provided by PVA cryogels.

In general, the modulus of the PVA cryogel increases with the number offreeze-thaw cycles. In one experimental series, thermally cycled PVAcryogels had compressive moduli in the range of 1-18 MPa and shearmoduli in the range of 0.1-0.4 MPa (see Stammen, J. A., et al.,Mechanical properties of a novel PVA hydrogel in shear and unconfinedcompression Biomaterials, 2001 22: p. 799-806).

As cryogels are crosslinked by physical and not chemical means, there issome concern about their structural stability. The modulus of PVA inaqueous solution increases with soak time in distilled water at constanttemperature. In one experiment, conducted over 40 days, the modulusincreased by 50%. Theoretically, during aqueous aging, the increase instrength, with the concomitant loss of soluble PVA, is the result of anincrease in the order of the supramolecular packing of the polymerchains.

It is also important to understand the effects of loss of polymer overtime and how that impacts the local host biological environment. Itshould be noted that in this example, the cryogel was only freeze-thawcycled once, although others have shown PVA dissolution followingmultiple freeze-thaw cycles. In general, there is very littleinformation about the stability of PVA cryogel modulus under repeatedload cycling (fatigue).

As might be expected, the swelling of PVA cryogels at any time pointdecreases with increasing number of freeze-thaw cycles, indicating adensification of the PVA gel, most likely due to a higher crosslinkdensity. In the long term, following gelation and under staticconditions, the ultimate swelling ratio decreases while the modulusincreases with time. In freeze-thaw processing, temperature is used toforce a phase separation of the PVA solution, thus enhancing thegelation mechanism in the PVA (it should be noted that even at roomtemperature a solution of PVA begins to gel weakly over time).

When PVA in aqueous solution (or in aqueous/DMSO mixtures) is heated todissolution and then frozen and thawed repeatedly, it forms a highlyelastic gel. The solgel transition forms a physically (not chemically)crosslinked polymer. Thus, the crosslinking that is achieved isthermo-reversible. There is a dependence of the cryogel characteristicson the molecular weight of the uncrosslinked polymer, the concentrationof the aqueous solution, temperature and time of freezing, theheating/cooling rates and the number of freeze-thaw cycles. Thus, thereis a rich parameter space from which control of the mechanicalproperties of the PVA cryogels may be exercised. PVA cryogels exhibitvery low toxicity (at least partially due to their low surface energy),contain few impurities and their water content can be made commensurateto tissue at 80 to 90 wt % and are thus generally considered to befairly biocompatible.

Pores can increase in size with the number of freezing-thawing cycles.It is thought that the polyvinyl polymer is rejected from the icecrystals as an impurity and is progressively “volume excluded” intoincreasingly polyvinyl polymer rich areas. As might be expected, thepore size increases with decreasing concentration of polyvinyl polymer.

The melting point for freeze-thaw cycled cryogels in pure aqueoussolutions is about 70-80° C. The melting point of a PVA cryogel inwater/dimethyl sulfoxide (DMSO) solutions increases with the number offreeze thaw cycles. For a 10-30% concentration of DMSO in water, themelting point increased with an increase in freezing time. Quantifyingthe complex relationship between the melting point as a function of thefreezing time, the concentration of DMSO, the concentration of the PVAand the number of freeze-thaw cycles is difficult. In general, themelting point increased with PVA concentration and with the number offreeze thaw cycles. In FIG. 3, the melting point variation as a functionof PVA concentration and the number of freeze thaw cycles is shown forPVA in a 1% DMSO/water solution. FIG. 3 is a graphic illustration of thedependence of melting temperature on polymer concentration, with afamily of curves for different numbers of freeze-thaw cycles forcryogels in 1 vol % DMSO at −40° C., where open circles represent datafrom gels treated with one cycle, closed circles represent data fromgels treated with three cycles, open triangles represent data from gelstreated with four cycles, closed triangles represent data from gelstreated with eight cycles and open squares represent data from gelstreated with fourteen cycles.

Because of the increased interaction between the PVA molecules and thesolvent across a range of DMSO solvent concentrations (20-30% vol), themelting point of the PVA is extremely low (near or below 10° C.). Ingeneral, the melting point increases with the number of freeze/thawcycles and increasing PVA concentration. At very high concentrations ofDMSO (90%), the cryogels have a very low melting point and weretransparent. After the first freeze/thaw cycle, the melting point doesnot change appreciably. The melting temperature of PVA cryogels in lowconcentration DMSO (1-5%) is independent of freezing time. However, themelting temperature of PVA in 30% DMSO is strongly dependent on freezingtime. This dependence is probably due to retarded freezing in higherconcentrations of DMSO. Faster freezing reduces the effects of crystalmovement on the formation of cross-links. As a consequence, the meltingpoint of PVA frozen quickly, and then held for longer periods of time(low concentration of DMSO) is lower than for PVA that does not freezequickly (high concentration of DMSO). At higher concentrations of PVA,the melting point dependence on freezing time in higher concentration ofDMSO is not as marked. However, the melting point is already very highfor these samples. The highest melting points for PVA/DMSO/Watercryogels are found in gels that do not have frozen water in them duringthe “freeze” (40-80% DMSO).

Effect of Thawing Rate

Gel-fraction measurements of aqueous solutions of PVA demonstrate thatslower thawing leads to less leachable polymer. The data corroboratesthe observation of a more efficient gelation process with decreasingthaw rates. The shear modulus of the hydrogel increases approximatelylinearly with decreasing log of the thawing rate (FIG. 4). FIG. 4 is agraphic illustration of the dependence of the shear modulus on the logof the thawing rate for PVA hydrogels formed by a single freeze-thawcycle of 7 g/dl solution of PVA in water (data from Yamaura, K., et al.,Properties of gels obtained by freezing/thawing of poly(vinylalcohol)/water/dimethyl sulfoxide solutions. Journal of Applied PolymerScience 1989 37:2709-2718). Low thaw rates of 0.02° C./min generatecryogels with shear moduli of 10.55 kPa for a 10 g/dL concentration ofPVA. No gelling occurred in samples thawed at 10° C./min. The loss ofsoluble polymer in aqueous media is decreased when the initial polymerconcentration is high (˜12 g/dL) provided that the thawing rate is low(˜0.02° C./min).

Modulus

In general, the modulus of the PVA cryogel increases with the number offreeze-thaw cycles. The freeze-thaw effect has been exploited togenerate PVA cryogels with fairly high moduli. In an experimental seriesaimed at determining whether PVA cryogels could be used in load bearingapplications (i.e. cartilage), thermally cycled PVA cryogels hadcompressive moduli in the range of 1-18 MPa (at very high strain) andshear moduli in the range of 0.1-0.4 MPa. The material used in thisseries of experiments is Salubria™ (available from SaluMedica, Atlanta,Ga.).

Modulus Stability.

Due to the thermoreversible nature of cryogels there has been concern inthe literature about the stability of the crosslinks. It has beenobserved that the modulus of non-cryogel PVA in aqueous solutionincreases with soak time in distilled water at a constant temperature.In one experiment, conducted over 40 days, the modulus increased 1.5times. It is possible that during aqueous aging, the increase instrength with the concomitant loss of soluble PVA is the result of anincrease in the order of the supramolecular packing of the polymerchains; in other words, even at moderate temperatures, there is a weakgelation process. There are significant implications in these data forlong term storage of freeze-thaw gelled PVA. It is also important tounderstand the effects of loss of polymer over time and how that impactsthe local host biological environment.

Swelling.

As might be expected, the swelling of PVA cryogels at any time pointdecreases with increasing number of freeze-thaw cycles, while thestorage modulus of PVA increases with the number of freeze-thaw cycles.However, following gelation and under static conditions, the ultimateswelling ratio decreases while the modulus increases with time. Theseobservations are consistent with the theory of residual soluble polymerleaching out, proposed by Lozinsky et al. The swelling dynamics of PVAcryogels typically obey the square root law (swelling ratio vs immersiontime) that is characteristic of a diffusion process.

PVA gels may also be produced through thermal cycling (not necessarilywith freezing) with dehydration. Such gels are potentially suitable foruse in load bearing applications, specifically, for use as an artificialarticular cartilage. In such applications, an artificial cartilage canbe made from PVA with a high degree of polymerization (7000), whichtranslates to an average molecular weight of 308,000 g/mol. To generatehigh modulus PVA from this polymer, the polymer powder is dissolved in amixture of water and DMSO. The solution is cooled to below roomtemperature to obtain a transparent gel. The gel is then dried using avacuum dehydrator for 24 hours at room temperature and then heat treatedin a silicone oil bath for 1 hour at 140° C. The PVA is placed in wateruntil maximum hydration was achieved. The water content can becontrolled by varying the annealing, or heat-treating, process. Theresulting PVA hydrogel can have a water content of approximately 20%,which is low.

Examination of the material properties of this thermally cycled PVAfound that the material distributes stress more homogeneously than stiffbiomaterials (UHMWPE) and preserves the lubrication film gap readily insimulated articular cartilage loading. The material sustained anddistributed pressure in the thin film of between 1 and 1.5 MPa. Intransient load tests, the PVA withstood and distributed loads of nearly5 MPa (FIG. 5). FIG. 5 is a graphic illustration of the time course oftransient stresses transmitted through samples (mass≈27 N) of variousmaterials dropped from a height of 10 mm, where curve 1 is polyethylene,curve 2 is subchondral bone with articular cartilage, curve 3 issubchondral bone without articular cartilage, and curve 4 is a 20%aqueous PVA hydrogel; data are from Lozinsky, V. I. and Damshkaln, L.G., Study of cryostructuration of polymer systems. XVII. Poly(vinylalcohol) cryogels: Dynamics of cryotropic gel formation. Journal ofApplied Polymer Science 2000 77:2017-2023.

Oka and colleagues further examined the wear properties of thermallycycled PVA under a variety of conditions (Oka, M, et al., Development ofartificial articular cartilage, Pro. Inst. Mech. Eng. 2000 214:59-68).The wear factor found in unidirectional pin-on-disk (against alumina)experiments is comparable to that of UHMWPE. However, in reciprocatingtests, the wear factor is up to 18 times larger. To improve the wearproperties, PVA of even higher molecular weight and cross-linked byγ-radiation (doses over 50 kGy) were used. Such treatment reduces thewear factor considerably (to about 7 times that of UHMWPE).

SUMMARY OF THE INVENTION

The present invention provides methods of making covalently crosslinkedvinyl polymer hydrogels having advantageous physical properties. Inother embodiments, the present invention provides covalently crosslinkedvinyl polymer hydrogel compositions made by a method of the presentinvention. In further embodiments, the present invention providesarticles of manufacture comprising covalently crosslinked vinyl polymerhydrogel compositions made by a method of the present invention.

In preferred embodiments, covalently cross-linked vinyl polymerhydrogels are produced by making a physically associated vinyl polymerhydrogel having a crystalline phase, exposing the physically associatedvinyl polymer hydrogel to an amount of ionizing radiation providing aradiation dose that is effective to form covalent crosslinks, andremoving physical associations by exposing the irradiated vinyl polymerhydrogel to a temperature above the melting point of the physicallyassociated crystalline phase to produce a covalently cross-linked vinylpolymer. Typically the radiation dose is in the range of about 1-1,000kGy. The physical properties of the produced hydrogel can be adjusted byvarying controlled parameters such as the proportion of physicalassociations, the concentration of polymer and the amount of radiationapplied. Such covalently crosslinked vinyl polymer hydrogels can be madetranslucent, preferably transparent, or opaque depending on theprocessing conditions. The stability of the physical properties of theproduced vinyl polymer hydrogel can be enhanced by controlling theamount of covalent crosslinks. Preferably the fraction of physicalassociations removed ranges from about one tenth to substantially all ofthe physical associations. In other preferred embodiments, about 1-90%of the physical associations are removed.

In accordance with a preferred embodiment, the method of manufacturing acovalently crosslinked vinyl polymer hydrogel includes the steps ofproviding a vinyl polymer solution comprising a vinyl polymer dissolvedin a solvent; heating the vinyl polymer solution to a temperatureelevated above the melting point of the physical associations of thevinyl polymer, inducing gelation of the vinyl polymer solution;controlling the gelation rate to form crystalline physical associationsin the vinyl polymer hydrogel, exposing the physically associated vinylpolymer hydrogel to a dose of ionizing radiation of about 1-1,000 kGyeffective to produce covalent crosslinks and melting the vinyl polymerhydrogel in a solvent to remove substantially all or a fraction of thephysical associations. In some preferred embodiments, the producedcovalently crosslinked vinyl polymer hydrogel substantially lacksphysical associations.

The desired physical property typically includes at least one of lighttransmission, gravimetric swell ratio, shear modulus, load modulus, lossmodulus, storage modulus, dynamic modulus, compressive modulus,crosslinking and pore size.

In preferred embodiments, the vinyl polymer is selected from the groupconsisting of poly(vinyl alcohol), poly(vinyl acetate), poly(vinylbutyral), poly(vinyl pyrrolidone) and a mixture thereof. Preferably thevinyl polymer is highly hydrolyzed poly(vinyl alcohol) of about 50kg/mol to about 300 kg/mol molecular weight. In preferred embodiments,the vinyl polymer is highly hydrolyzed poly(vinyl alcohol) of about 100kg/mol molecular weight. Typically the vinyl polymer solution is about0.5-50 weight percent solution of poly(vinyl alcohol) based on theweight of the solution. In certain preferred embodiments, the vinylpolymer solution is about 1-15 weight percent. In other preferredembodiments, the vinyl polymer solution about 10-20 weight percentpolyvinyl alcohol. The vinyl polymer, preferably poly(vinyl alcohol),can be isotactic, syndiotactic or atactic.

The solvent of the vinyl polymer solution is selected from the groupconsisting of deionized water (DI), methanol, ethanol, dimethylsulfoxide and a mixture thereof. The solvent used in melting the vinylpolymer hydrogel to remove the physical associations is selected fromthe group consisting of deionized water, methanol, ethanol, dimethylsulfoxide and a mixture thereof. In preferred embodiments, the samesolvent is used for the vinyl polymer solution and for melting the vinylpolymer hydrogel to remove the physical associations.

In preferred embodiments, the ionizing radiation is gamma radiation orbeta particles (electron beam). In preferred embodiments, the totalradiation dose is suitably 1-1,000 kGy, preferably 50-1,000 kGy, morepreferably 10-200 kGy. The radiation dose rate is suitably about 0.1-25kGy/min, preferably about 1-10 kGy/min. In preferred embodiments, theirradiation dose used is within 20% of the optimum irradiation dose,preferably within 10%, more preferably within 7% of the optimumirradiation dose. The optimum irradiation dose is specific to eachpolymer.

In preferred embodiments, the suitable polymer concentration of thehydrogel product to be irradiated can be optimized within the polymerconcentration range flanking the maximum of a plot of intermolecularcrosslinking yield v. polymer concentration or the minimum of a plot ofirradiation dose v. polymer concentration, i.e. the point at which theslope of the plot is zero. Suitably, the polymer concentration falls ina range in which the intermolecular crosslinking yield or theirradiation dose is within 20% of the maximum or minimum value,respectively, preferably within 10%, more preferably within 7% of thevalue. Where the hydrogel comprises poly(vinyl alcohol), the hydrogel issuitably about 2 to about 35 weight percent poly(vinyl alcohol),preferably about 3.5 to about 30 weight percent poly(vinyl alcohol),more preferably about 5 to about 25 weight percent poly(vinyl alcohol),based on the weight of the composition.

After irradiation, the physical associations are removed by raising thetemperature of the hydrogel above the melting point of thethermo-reversible physical associations. The required temperaturedepends on the melting point of the cross-links and is suitably about0-100 degrees Celsius, preferably about 40-80 degrees Celsius.Preferably the irradiated gels are heated to high temperatures whilethey are immersed in solvent to allow dissolution and elution of the PVAchains “melted out” of the physical associations. The duration of theexposure to the elevated temperature can be adjusted to melt out all ofthe physical associations, or just a fraction of the physicalassociations.

The covalently crosslinked vinyl polymer hydrogels of the presentinvention have an advantageous inherent material stability that isexhibited when the crosslinking is covalent chemical rather thanphysical. Forming covalent crosslinks by radiation rather than bychemical reagents avoids the potential problem of residual contaminants.For medical materials and articles of manufacture, both the irradiationand the sterilization steps can be performed simultaneously, simplifyingmanufacturing and reducing costs. The ability to control pore size byvarying the degree of precursor gel physical crosslinking will be anadvantage over other means of forming covalent vinyl polymer hydrogels.

The methods are applicable to the creation of materials for use inmedical, biological and industrial areas including the controlleddelivery of agents (which may include proteins, peptides,polysaccharides, genes, DNA, antisense to DNA, ribozymes, hormones,growth factors, a wide range of drugs, imaging agents for CAT, SPECT,x-ray, fluoroscopy, PET, MRI and ultrasound), generation of load bearingimplants for hip, spine, knee, elbow, shoulder, wrist, hand, ankle, footand jaw, generation of a variety of other medical implants and devices(which may include active bandages, trans-epithelial drug deliverydevices, sponges, anti-adhesion materials, artificial vitreous humor,contact lens, breast implants, stents and artificial cartilage that isnot load bearing (i.e., ear and nose)), any application where gradients(single or multiple) in mechanical properties or structure are required.

The foregoing and other features and advantages of the system and methodfor PVA hydrogels will be apparent from the following more particulardescription of preferred embodiments of the system and method asillustrated in the accompanying drawings in which like referencecharacters refer to the same parts throughout the different views.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a graphic illustration of the efficiency of a dose of gammairradiation measured as the intermolecular crosslinking yield Gx(10⁻⁷mol.J⁻¹) as a function of PVA concentration, as reported in Wang,S., et al., The influence of polymer concentration on theradiation-chemical yield of intermolecular crosslinking of poly(vinylalcohol) by gamma-rays in deoxygenated aqueous solution. RadiationPhysics and Chemistry 2000 59: 91-95. FIG. 1B is a graphic illustrationof the efficiency of a dose of gamma irradiation measured as theintermolecular crosslinking yield Gx (10⁻⁷mol.J⁻¹) as a function of thepolymer concentration in the lower concentration range of 0-100 g dm⁻³.

FIG. 2 is a graphic illustration of the gelation dose of gammairradiation as a function of poly(vinyl pyrrolidone) concentration, asreported in Rosiak, J. M. & Ulanski, P., Synthesis of hydrogels byirradiation of polymers in aqueous solution, Radiation Physics andChemistry 1999 55: 139-151. The local minimum in the range 10-40 g dm⁻³indicates an ideal polymer concentration. The roll-off at higherconcentrations is due to overlapping polymer domains which restrictmovement of the chains and limit the diffusion of radicals, causingchain scission instead of crosslinking. At lower concentrations, thegelling dosage increases rapidly with decreasing concentration;intramolecular cross-linking dominates because the distance betweenmolecules is too large to facilitate intermolecular crosslinking at lowpolymer concentrations.

FIG. 3 is a graphic illustration of the dependence of meltingtemperature on polymer concentration, with a family of curves fordifferent numbers of freeze-thaw cycles for cryogels in 1 vol % DMSO at−40° C., where open circles represent data from gels treated with onecycle, closed circles represent data from gels treated with threecycles, open triangles represent data from gels treated with fourcycles, closed triangles represent data from gels treated with eightcycles and open squares represent data from gels treated with fourteencycles.

FIG. 4 is a graphic illustration of the dependence of the shear moduluson the log of the thawing rate for PVA hydrogels formed by a singlefreeze-thaw cycle of 7 g/dl solution of PVA in water, data from Yamaura,K., et al., Properties of gels obtained by freezing/thawing ofpoly(vinyl alcohol)/water/dimethyl sulfoxide solutions. Journal ofApplied Polymer Science 1989 37:2709-2718.

FIG. 5 is a graphic illustration of the time course of transientstresses transmitted through samples (mass=27 N) of various materialsdropped from a height of 10 mm, where curve 1 is polyethylene, curve 2is subchondral bone with articular cartilage, curve 3 is subchondralbone without articular cartilage, and curve 4 is a 20% aqueous PVAhydrogel; data are from Lozinsky, V. I. and Damshkaln, L. G., Study ofcryostructuration of polymer systems. XVII. Poly(vinyl alcohol)cryogels: Dynamics of cryotropic gel formation. Journal of AppliedPolymer Science 2000 77:2017-2023.

FIG. 6 is a flow chart 100 of a preferred embodiment of the method ofthe present invention, showing the steps of providing a physicallyassociated hydrogel 110, exposing the physically associated hydrogel toionizing radiation to form covalent crosslinks 112 and removing physicalassociations 114.

FIG. 7 is a flow chart 150 of another preferred embodiment of the methodof the present invention, showing the steps of providing a vinyl polymersolution 152, heating the vinylpolymer solution above the melting pointof physical associations 156, inducing gelation 160, controlling thegelation rate to form crystalline physical associations 166, exposingthe physically associated hydrogel to ionizing radiation to formcovalent crosslinks 170 and removing physical associations 180.

FIG. 8 is a graphic illustration of the results of dynamic mechanicalanalysis of 10-20 weight percent aqueous PVA hydrogels (10⁻⁵ g/mole,93%+hydrolyzed) cast as thin (4 mm) sheets and subjected to onefreeze-thaw cycle by immersion in a NaCl/ice bath at −21 degrees Celsiusfor eight hours and then allowing them to thaw at room temperature forfour hours in accordance with a preferred embodiment of the presentinvention. The samples were then irradiated in a hydrated state to 0,25, or 100 kGy with an electron beam. Some of the resultant gels werethen raised to 80° C. to melt the crystals generated by the freeze-thawcycle. Dynamic mechanical analysis was conducted at 37° C. at 1 Hz indistilled water. Group 1—control (0 kGy), Group 2—25 kGy, Group 3—100kGy. The drop in storage modulus for the “melt” samples is ascribed tothe loss of thermally reversible physical associations due to freezethawing. No melt data is shown for the 0 kGy irradiated samples sincethese materials completely disassociated upon melting.

FIG. 9 is a graphic illustration of the results of dynamic mechanicalanalysis of 10-20 weight percent aqueous PVA hydrogels (10⁻⁵ g/mole,93%+hydrolyzed) cast as thin (4 mm) sheets and subjected to fourfreeze-thaw cycles by immersion in a NaCl/ice bath at −21 degreesCelsius for eight hours and then allowing them to thaw at roomtemperature for four hours in accordance with a preferred embodiment ofthe present invention. The samples were then irradiated in a hydratedstate to 0, 25, or 100 kGy with an electron beam. Some of the resultantgels were then raised to 80° C. to melt the crystals generated by thefreeze-thaw cycle. Dynamic mechanical analysis was conducted at 37° C.at 1 Hz in distilled water. Group 1—control (0 kGy), Group 2—25 kGy,Group 3—100 kGy. The drop in storage modulus for the “melt” samples isascribed to the loss of thermally reversible physical associations dueto freeze thawing. No melt data is shown for the 0 kGy irradiatedsamples since these materials completely disassociated upon melting.

FIG. 10 shows an array 200 of four cylindrical PVA hydrogels 210, 220,230 and 240 comprising 10% PVA formed by a single freeze-thaw cycle.Solutions of poly(vinyl alcohol) (105 g/mole; 93%+hydrolyzed) wereprepared in water to concentrations of 10%. The solutions were cast inthin sheets (4 mm) and subjected to one freeze-thaw cycle by immersionin a NaCl/ice bath at −21° C. for 8 hours and then allowed to thaw atroom temperature. Cylindrical disk samples were cut from the sheets.

FIG. 11 shows an array 250 of two cylindrical PVA hydrogels 260 and 270comprising 10% PVA formed by a single freeze-thaw cycle followed byirradiation, in accordance with a preferred embodiment of the presentinvention. Ten percent aqueous solutions of poly(vinyl alcohol) (10⁵g/mole; 93%+hydrolyzed) were prepared. The solutions were cast in thinsheets (4 mm) and subjected to one freeze-thaw cycle by immersion in aNaCl/ice bath at −21° C. for 8 hours and then allowed to thaw at roomtemperature. Cylindrical disk samples were cut from the sheets. Thesamples were then irradiated in a hydrated state to 100 kGy by anelectron beam.

FIG. 12 shows a coil 300 formed of a PVA hydrogel 310 comprising 10% PVAshowing retention of the coiled form after the melting-out of physicalassociations. Solutions of poly(vinyl alcohol) (10⁵ g/mole;93%+hydrolyzed) were prepared in water to concentrations of 10 wt. %.The solutions were poured into flexible tubing with interior diametersof 0.25″. The ends of each piece of tubing were sealed, the tubes werecoiled into a spiral, and the spirals were subjected to one freeze-thawcycle by immersing in an NaCl/ice bath at −21 ° C. for 8 hours and thenallowing them to thaw at room temperature for four hours. The sampleswere then irradiated in a hydrated state to 100 kGy with an electronbeam. Some of the resultant coiled gels were then raised to 80° C. tomelt the freeze-thaw generated associations. The coiled gels could bestretched into a straight rod, but resumed their coiled state uponrelease of the applied tension. Suitably size coils can be insertedthrough a cannula or lumen into the intervertebral space to replace anucleus pulposus.

FIG. 13 shows an array 400 of packaged PVA disks about to undergoelectron beam irradiation where disks 410, 420 received no shielding,disks 430, 440 received gradient shielding and disks 450, 460 receivedstepped shielding.

FIGS. 14A and 14B are an illustration 500 showing the effects ofirradiation using a continuous gradient mask. FIG. 14A shows acontinuous gradient PVA hydrogel 510 formed by a single freeze-thawcycle and then irradiated in a hydrated state to 100 kGy with anelectron beam prior to melt-out. The arrow 520 points in the directionof increasing covalent crosslinks (higher received dose). FIG. 14B showsthe same continuous gradient PVA hydrogel 510 shown in FIG. 13Afollowing melt-out having the arrow 520 pointing in the direction ofincreasing covalent crosslinks (higher received dose), where the boxes530, 540 indicate the locations where the swelling ratio was assessed.Note the increase in transparency following melt-out of physicalassociations.

FIGS. 15A and 15B are an illustration 600 showing the effects ofirradiation using a stepped gradient mask. FIG. 15A shows a steppedgradient PVA hydrogel 610 formed by a single freeze-thaw cycle and thenirradiated in a hydrated state to 100 kGy with an electron beam prior tomelt-out. The circle 620 indicates the position during irradiation ofthe mask (an aluminum disk). FIG. 15B shows the same stepped gradientPVA hydrogel 610 shown in FIG. 14A following melt-out, where the boxes630, 640 indicate the locations where the swelling ratio was assessed.

FIG. 16 shows an array 700 of PVA hydrogels prior to irradiation andmelt-out, where sample 710 is a 10% PVA hydrogel formed by a singlefreeze-thaw cycle, sample 720 is a 20% PVA hydrogel formed by a singlefreeze-thaw cycle, sample 730 is a 10% PVA hydrogel formed by fourfreeze-thaw cycles, sample 740 is a 20% PVA hydrogel formed by fourfreeze-thaw cycles, and a U.S. penny 750 is provided for scale.

FIG. 17 shows an array 800 of PVA hydrogels after irradiation andmelt-out (immersion in deionized water at 80° C.), where sample 810 is a10% PVA hydrogel formed by a single freeze-thaw cycle and irradiated to25 kGy, sample 820 is a 20% PVA hydrogel formed by a single freeze-thawcycle and irradiated to 25 kGy, sample 830 is a 10% PVA hydrogel formedby a single freeze-thaw cycle and irradiated to 100 kGy, sample 840 is a20% PVA hydrogel formed by a single freeze-thaw cycle and irradiated to100 kGy, sample 850 is a 10% PVA hydrogel formed by four freeze-thawcycles and irradiated to 25 kGy, sample 860 is a 20% PVA hydrogel formedby four freeze-thaw cycles and irradiated to 25 kGy, sample 840 is a 10%PVA hydrogel formed by four freeze-thaw cycles and irradiated to 100kGy, and sample 880 is a 20% PVA hydrogel formed by four freeze-thawcycles and irradiated to 100 kGy.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Covalently cross-linked poly(vinyl alcohol) (PVA) gels can be producedby making a physically associated PVA hydrogel that has a crystallinephase, forming covalent crosslinks by exposing the physically associatedPVA hydrogel to an effective amount of ionizing radiation, and removingthe physical associations by exposure to a temperature above the meltingpoint of the physically associated crystalline phase to produce acovalently cross-linked vinyl polymer hydrogel. The physical propertiesof the produced hydrogel can be adjusted by varying controlledparameters such as the proportion of physical associations, theconcentration of polymer and the amount of radiation applied. PVAcovalently cross-linked vinyl polymer hydrogels can be made translucent,preferably transparent, or opaque depending on the processingconditions. The stability of the physical properties of the producedhydrogel can be enhanced by controlling the amount of covalentcrosslinks.

Such PVA hydrogels can be made to have a wide range of mechanicalproperties, such as very low to moderately high compressive moduli.Critical to the final modulus is the number of physical associationspresent in the precursor gels. A large number of physical associationsserves to reduce the total yield of the radiation induced crosslinks,reducing the final modulus of the material. Thus, weakly associatedprecursor physical gels produce stronger covalently cross-linked vinylpolymer hydrogels. This phenomenon allows control of the final materialproperties by modulation of the physical associations in the precursorgel.

The porosity and pore size in covalently cross-linked vinyl polymerhydrogels can be controlled in that the melt-out step removes physicalassociations, leaving voids of controllable volume. This is not possibleby direct irradiation of PVA solutions. In addition, upon completion ofthe processing, they will be inherently sterile due to the irradiationprocessing.

Polyvinyl alcohols are commonly divided into “fully hydrolyzed” and“partly hydrolyzed” types, depending on how many mole-percent ofresidual acetate groups remain in the molecule. Polyvinyl alcohols canbe manufactured from polyvinyl acetate by alcoholysis using a continuousprocess. By varying the degree of polymerization of the polyvinylacetate and its degree of hydrolysis (saponification) a number ofdifferent grades can be supplied. Typically, suitable polyvinyl alcoholsfor the practice of the present invention have a degree of hydrolysis(saponification) of about 80-100 percent, preferably about 95-99.8percent. The degree of polymerization of suitable polyvinyl alcohols forthe practice of the present invention is in the range of about 100 toabout 50,000, preferably about 1,000 to about 20,000.

Crosslinks in PVA gels may be either covalent (chemical) crosslinks orphysical associations (physical). Covalent crosslinks are formedtypically through chemical modification, or through irradiation.Physical associations may be formed via freeze-thaw cycling, dehydrationor through controlled manipulation of the solubility of the vinylpolymer in a solvent (to produce a “thetagel”), disclosed in U.S.published patent application US20040092653 or by a combination of suchmethods. In general, the formation of a thetagel includes a step ofmixing the vinyl polymer solution with a gellant, wherein the resultingmixture has a higher Flory interaction parameter than the vinyl polymersolution. In the present invention, both covalent and physicalassociations can be employed, in that a physically cross-linkedprecursor gel will be covalently crosslinked by irradiation.

The use of irradiation to form covalent crosslinks has severaladvantages over chemical crosslinking. Chemical crosslinking is oftenperformed by the addition of a reactive metallic salt or aldehyde andsubjecting the system to thermal radiation. For example, crosslinkingmay be performed by adding (di-) isocyanates,urea-/phenolic-melamine-resins, epoxies, or (poly-)aldehydes. However,the use of such reagents for chemical crosslinking can leave residuesthat decrease the biocompatibility of the PVA hydrogel.

Crosslink formation by irradiation of polymers in solution is a suitablemethod for the generation of hydrogels for biomedical use. Crosslinkingvia an ionization source provides adequate control of the reaction, alower number of unwanted processes (e.g. homografting of monomer to theside of a polymer chain) and generates an end product suitable for usewith little additional processing or purification. The irradiation andsterilization steps can often be combined.

FIG. 6 is a flow chart of a preferred embodiment of the method of thepresent invention. In a preferred embodiment, the present inventionprovides a method of making a covalently cross-linked vinyl polymerhydrogel comprising the steps of providing a physically crosslinkedvinyl polymer hydrogel having a crystalline phase; exposing thephysically crosslinked vinyl polymer hydrogel to an amount of ionizingradiation providing a radiation dose in the range of about 1-1,000 kGyeffective to form covalent crosslinks; and removing the physicalassociations by exposing the irradiated vinyl polymer hydrogel to atemperature above the melting point of the physically associatedcrystalline phase to produce a covalently cross-linked vinyl polymerhydrogel. In preferred embodiments, the step of providing a physicallyassociated vinyl polymer hydrogel having a crystalline phase includesthe steps of providing a vinyl polymer solution comprising a vinylpolymer dissolved in a solvent; heating the vinyl polymer solution to atemperature elevated above the melting point of the physicalassociations of the vinyl polymer; inducing gelation of the vinylpolymer solution; and controlling the gelation rate to form physicalassociations in the vinyl polymer hydrogel. In preferred embodiments,the vinyl polymer is selected from the group consisting of poly(vinylalcohol), poly(vinyl acetate), poly(vinyl butyral), poly(vinylpyrrolidone) and a mixture thereof. Preferably, the vinyl polymer ispoly(vinyl alcohol).

In preferred embodiments, the solvent of the vinyl polymer solution isselected from the group consisting of deionized water, methanol,ethanol, dimethyl sulfoxide and a mixture thereof. In preferredembodiments, the irradiated vinyl polymer hydrogel is immersed in asolvent is selected from the group consisting of deionized water,methanol, ethanol, dimethyl sulfoxide and a mixture thereof while isexposed to a temperature above the melting point.

Solutions of poly(vinyl alcohol) (10⁵ g/mole; 93%+hydrolyzed) wereprepared in water to concentrations of 10% to 20%. The solutions werecast in thin sheets (4 mm) and subject to one cycle by immersion in aNaCl/ice bath at −21° C. for eight hours and then allowing them to thawat room temperature for four hours. The samples were then irradiated ina hydrated state to 0, 25, or 100 kGy with electron beam. Some of theresultant gels were then raised to 80° C. to melt the freeze-thawgenerated crystals. Dynamic mechanical analysis was conducted at 37° C.at 1 Hz in distilled water. The resulting storage moduli for the DMAtests are shown in FIG. 7. FIG. 7 is a graphic illustration of theresults of dynamic mechanical analysis of 10-20 weight percent aqueousPVA hydrogels (10⁻⁵ g/mole, 93%+hydrolyzed) cast as thin (4 mm) sheetsand subjected to one freeze-thaw cycle by immersion in a NaCl/ice bathat −21 degrees Celsius for eight hours and then allowing them to thaw atroom temperature for four hours in accordance with a preferredembodiment of the present invention. The samples were then irradiated ina hydrated state to 0, 25, or 100 kGy with an electron beam. Some of theresultant gels were then raised to 80° C. to melt the crystals generatedby the freeze-thaw cycle. Dynamic mechanical analysis was conducted at37° C. at 1 Hz in distilled water. Group 1—control (0 kGy), Group 2—25kGy, Group 3—100 kGy. The drop in storage modulus for the “melt” samplesis ascribed to the loss of thermally reversible physical associationsdue to freeze thawing. No melt data is shown for the 0 kGy irradiatedsamples since these materials completely disassociated upon melting.

The results indicate that progressively stronger irradiation ofprecursor gels increases the compressive modulus. However, followingmelt-out of the physical associations, the gels have a lower modulusthan the precursor gel if they are not irradiated with at least 100 kGy.Following melt out, there is no difference in the mechanical propertiesof the covalently cross-linked vinyl polymer hydrogels that started withprecursor gels of 10 or 20% PVA.

DMA Testing, Strongly Cross-Linked Precursor Gels—Method

Solutions of poly(vinyl alcohol) (10⁵ g/mole; 93%+hydrolyzed) wereprepared in water to concentrations of between 10% and 20%. Thesolutions were cast in thin sheets (4 mm) and subject to four cycles offreeze-thaw by immersion in a NaCl/ice bath at −21° C. for eight hoursand then allowing them to thaw at room temperature for four hours priorto the next cycle. The samples were then irradiated in a hydrated stateto 0, 25, or 100 kGy with electron beam. Some of the resultant gels werethen raised to 80° C. to melt the freeze-thaw generated crystals.Dynamic mechanical analysis was conducted at 37° C. at 1 Hz in distilledwater.

The resulting storage moduli for the DMA tests are shown in FIG. 8. FIG.8 is a graphic illustration of the results of dynamic mechanicalanalysis of 10-20 weight percent aqueous PVA hydrogels (10⁻⁵ g/mole,93%+hydrolyzed) cast as thin (4 mm) sheets and subjected to fourfreeze-thaw cycles by immersion in a NaCl/ice bath at −21 degreesCelsius for eight hours and then allowing them to thaw at roomtemperature for four hours in accordance with a preferred embodiment ofthe present invention. The samples were then irradiated in a hydratedstate to 0, 25, or 100 kGy with an electron beam. Some of the resultantgels were then raised to 80° C. to melt the crystals generated by thefreeze-thaw cycle. Dynamic mechanical analysis was conducted at 37° C.at 1 Hz in distilled water. Group 1—control (0 kGy), Group 2—25 kGy,Group 3—100 kGy. The drop in storage modulus for the “melt” samples isascribed to the loss of thermally reversible physical associations dueto freeze thawing. No melt data is shown for the 0 kGy irradiatedsamples since these materials completely disassociated upon melting.

The data presented in this figure demonstrate that the number offreeze-thaw cycles (which can be correlated to the amount of PVAinvolved in physical associations) reduces the net effectiveness of theirradiation. A 10% PVA gel subjected to one freeze thaw cycle and 100kGy has a modulus of 200 kPa following melt-out (FIG. 7). The sameprocess performed on a precursor 10% PVA gel subjected to four freezethaw cycles yields a modulus of 90 kPa. Thus, the stronger thephysically associations in the precursor gel, the lower the yield ofchemical cross-links induced by radiation. This result also suggests thepossibility that gradient gels can be created by first generating agradient in physical associations (e.g. by differential dehydration ofPVA gels) and then subjecting the precursor gel to a uniformirradiation. The final gel will have gradient in cross-linking oppositein direction to that formed in the precursor gel.

EXAMPLE 1

Cylindrical covalently cross-linked vinyl polymer hydrogels wereirradiated using a uniform irradiation distribution. Solutions ofpoly(vinyl alcohol) (10⁵ g/mole; 93%+hydrolyzed) were prepared in waterto concentrations of 10%. The solutions were cast in thin sheets (4 mm)and subjected to one freeze-thaw cycle by immersion in an NaCl/ice bathat −21° C. for 8 hours and then allowing them to thaw at roomtemperature. Disks were cut from the sheets to form cylindrical disks.The samples were then irradiated in a hydrated state to 100 kGy byelectron beam. Some of the resultant gels were then raised to 80° C. tomelt-out the freeze-thaw generated crystals. Swelling ratios of the gelswere recorded to discern the effects of the processing on the cross-linkdensity.

FIGS. 9 and 10 show cylindrical PVA disks, generated by one freeze thawof 10% PVA solution, prior to and following irradiation. FIG. 9 shows anarray 100 of four cylindrical PVA hydrogels 110, 120, 130 and 140comprising 10% PVA formed by a single freeze-thaw cycle. Solutions ofpoly(vinyl alcohol) (105 g/mole; 93%+hydrolyzed) were prepared in waterto concentrations of 10%. The solutions were cast in thin sheets (4 mm)and subjected to one freeze-thaw cycle by immersion in an NaCl/ice bathat −21° C. for 8 hours and then allowing them to thaw at roomtemperature. Disks were cut from the sheets to form cylindrical disks.

FIG. 10 shows an array 200 of two cylindrical PVA hydrogels 210 and 220comprising 10% PVA formed by a single freeze-thaw cycle followed byirradiation, in accordance with a preferred embodiment of the presentinvention. Solutions of poly(vinyl alcohol) (10⁵ g/mole; 93%+hydrolyzed)were prepared in water to concentrations of 10%. The solutions were castin thin sheets (4 mm) and subjected to one freeze-thaw cycle byimmersion in an NaCl/ice bath at −21° C. for 8 hours and then allowingthem to thaw at room temperature. Disks were cut from the sheets to formcylindrical disks. The samples were then irradiated in a hydrated stateto 100 kGy by an electron beam.

Table 1 gives the gravimetric swell ratio for the gel prior to andfollowing the radiation melt-out procedure TABLE 1 PVA swelling ratioPrecursor hydrogel PVA 6.0 Covalently cross-linked vinyl 5.8 polymerhydrogels

Since there is virtually no change in swell ratio before and aftermelt-out, the process of irradiation and meltout of the PVA cryogelappears to exchange the heat labile physical associations for stablecovalent crosslinks. In this case, the original swelling ratio of thefinal gel is retained. This indicates that the melt-out served torelease the physical associations but did not result in much loss of PVAmaterial. Thus, there must have been substantial covalent crosslinkingto reinforce the weak physical crosslinking.

EXAMPLE 2

This example demonstrates that irradiation of freeze-thawed gels held ina particular shape induces “memory” of that shape. Solutions ofpoly(vinyl alcohol) (10⁵ g/mole; 93%+hydrolyzed) were prepared in waterto concentrations of 10 wt. %. The solutions were poured into flexibletubing with interior diameters of 0.25″. The ends of each piece oftubing were sealed, the tubes were coiled into a spiral, and the spiralswere subjected to one freeze-thaw cycle by immersing in a NaCl/ice bathat −21° C. for 8 hours and then allowing them to thaw at roomtemperature for four hours. The samples were then irradiated in ahydrated state to a radiation dose of 100 kGy using an electron beam.Some of the resultant coiled gels were then raised to 80° C. to melt thephysical associations produced by the freeze-thaw treatment.

FIG. 11 shows a coil 300 formed of a PVA hydrogel 310 comprising 10% PVAshowing retention of the coiled form after the melting-out of physicalassociations. Solutions of poly(vinyl alcohol) (10⁵ g/mole;93%+hydrolyzed) were prepared in water to concentrations of 10 wt. %.The solutions were poured into flexible tubing with interior diametersof 0.25″. The ends of each piece of tubing were sealed, the tubes werecoiled into a spiral, and the spirals were subjected to one freeze-thawcycle by immersing in an NaCl/ice bath at −21 ° C. for 8 hours and thenallowing them to thaw at room temperature for four hours. The sampleswere then irradiated in a hydrated state to 100 kGy with an electronbeam. Some of the resultant coiled gels were then raised to 80° C. tomelt the freeze-thaw generated associations. The coiled gels could bestretched into a straight rod, but resumed their coiled state uponrelease of the applied tension. Suitably size coils can be insertedthrough a cannula or lumen into the intervertebral space to replace anucleus pulposus.

EXAMPLE 3

The following examples demonstrate that both gradient and discreteshielding of the electron beam can be used to manipulate the finalproperties of the PVA covalently cross-linked vinyl polymer hydrogels.FIG. 12 shows three sets of PVA disks with various types of shielding toinduce spatial gradients in covalent crosslinking. FIG. 12 shows anarray 400 of packaged PVA disks about to undergo electron beamirradiation where disks 410, 420 received no shielding, disks 430, 440received gradient shielding and disks 450, 460 received steppedshielding. FIG. 13A shows a continuous gradient PVA hydrogel 510 formedby a single freeze-thaw cycle and then irradiated in a hydrated state to100 kGy with an electron beam prior to melt-out. The arrow 520 points inthe direction of increasing covalent crosslinks (higher received dose).FIG. 13B shows the same continuous gradient PVA hydrogel 510 shown inFIG. 13A following melt-out having the arrow 520 pointing in thedirection of increasing covalent crosslinks (higher received dose),where the boxes 530, 540 indicate the locations where the swelling ratiowas assessed. Note the increase in transparency following melt-out ofphysical associations. TABLE 2 Swell Ratio of Continuous GradientCovalently Cross-linked Vinyl Polymer Hydrogels Location of samplesSwell ratio Low shielding (location 530, FIG. 13B) 6.9 Medium shielding(location 540, FIG. 14B) 11.9 High shielding N/A (dissolved)

These results indicate that gradients in covalent crosslinking can beachieved using gradient shields during the electron beam process. Lessshielding results in higher cross-linking as indicated by the lowerswelling ratio.

EXAMPLE 4

A physically associated PVA hydrogel was irradiated while masked by acentrally placed aluminum disk to produce a step change in radiationdose between masked and exposed regions of the hydrogel. Shielding canbe utilized to crosslink PVA disks with a stepped difference inradiation crosslinks. In this embodiment the shielding is made from amaterial with a uniform density and thickness. In other embodiments,different locations of the shield can have different thickness ordifferent density and shaped to determine the area and degree of reducedradiation effectiveness. The material will block radiation (e-beam orgamma) in proportion to the thickness of the shielding piece. Followingradiation cross-linking, the gel can be held at high temperature tomelt-out the physical associations producing a PVA hydrogel having agradient of covalent crosslinks.

FIG. 14A shows a stepped gradient PVA hydrogel 610 formed by a singlefreeze-thaw cycle and then irradiated in a hydrated state to 100 kGywith an electron beam prior to melt-out. The circle 620 indicates theposition during irradiation of the mask (an aluminum disk). FIG. 14Bshows the same stepped gradient PVA hydrogel 610 shown in FIG. 14Afollowing melt-out, where the boxes 630, 640 indicate the locationswhere the swelling ratio was assessed. TABLE 3 Swell Ratios of SteppedGradient Covalently Cross-linked Vinyl Polymer Hydrogels Location ofsample Swelling ratio No shield (location 630, FIG. 14B) 5.9 Shielded(location 640, FIG. 14B) 12.2

This example demonstrates the ability to create sharp changes in thematerial properties of the covalently cross-linked vinyl polymerhydrogels by shielding with discrete, uniform materials. The unshieldedregion swells half as much as the shielded region indicating a sharpincrease in the number of covalent cross-links.

EXAMPLE 5

PVA covalently cross-linked vinyl polymer hydrogels suitable for use asmaterial for contact lenses were made. In general, freeze-thaw cryogelsproduced in aqueous solutions do not produce clear gels (FIG. 15). Inaddition, PVA gels are known to be poorly permeable (permeability is ageneral requirement of any contact lens material). FIG. 15 shows anarray 700 of PVA hydrogels prior to irradiation and melt-out, wheresample 710 is a 10% PVA hydrogel formed by a single freeze-thaw cycle,sample 720 is a 20% PVA hydrogel formed by a single freeze-thaw cycle,sample 730 is a 10% PVA hydrogel formed by four freeze-thaw cycles,sample 740 is a 20% PVA hydrogel formed by four freeze-thaw cycles, anda U.S. penny 750 is provided for scale. However, once the cryogels areirradiated and the physical associations are melted out, some of thembecome very transparent (FIG. 16). Also, because the bulky freeze-thawcrystals have been removed, their permeability should be greatlyenhanced as well. Such materials might be useful for contact lenses.Since several gels made by differing processing steps were transparent,it is likely that a lens with a range of porosities can be made. FIG. 16shows an array 800 of PVA hydrogels after irradiation and melt-out(immersion in deionized water at 80° C.), where sample 810 is a 10% PVAhydrogel formed by a single freeze-thaw cycle and irradiated to 25 kGy,sample 820 is a 20% PVA hydrogel formed by a single freeze-thaw cycleand irradiated to 25 kGy, sample 830 is a 10% PVA hydrogel formed by asingle freeze-thaw cycle and irradiated to 100 kGy, sample 840 is a 20%PVA hydrogel formed by a single freeze-thaw cycle and irradiated to 100kGy, sample 850 is a 10% PVA hydrogel formed by four freeze-thaw cyclesand irradiated to 25 kGy, sample 860 is a 20% PVA hydrogel formed byfour freeze-thaw cycles and irradiated to 25 kGy, sample 840 is a 10%PVA hydrogel formed by four freeze-thaw cycles and irradiated to 100kGy, and sample 880 is a 20% PVA hydrogel formed by four freeze-thawcycles and irradiated to 100 kGy.

Alternatively, gradient PVA covalently cross-linked vinyl polymerhydrogels can be made by first freeze-thawing a cylinder containing PVAsolution once, or a number of times. The resulting PVA cryogel cylinderscan be dehydrated in a variety of different ways (placed in warmsilicone oil, dried in a vacuum, dried in air at a controlled relativehumidity) such that the dehydration of the cryogel penetrates partiallyinto the cylinder causing a radial gradient in physical associations.The resulting material can then be sectioned perpendicular to its axisto make discs and irradiated before or after subsequent rehydration. Theresulting material (a gradient-dehydrated-freeze-thaw covalentlycross-linked vinyl polymer hydrogels ) should be soft on the outside andstiff in the middle. A nucleus implant created in this manner willspace-fill the inner disc and carry loads while transmitting acontrolled amount of that load to the annulus. The depth of moduluschange can be controlled by the length of exposure to dehydration. Forsamples dehydrated at varying relative humidities in air, the depth andthe ultimate modulus of the outer part of the gel can be controlled viaexposure time and humidity.

PVA hydrogels can be useful for drug delivery applications. A desireablecharacteristic of drug delivery materials is the ability to control thedrug release rate, often by controlling the pore size in the material.Typically a zero-order drug release rate is desired to eliminate bursteffects. To accomplish this, the typical approach is to create amaterial that restricts diffusion only at the surface interfacing thetissue to be treated. However, the present invention can provide theability to modulate not only pore size but gradients in pore size.Aqueous solutions of PVA are made as described above and cast into thinfilms. The cast gels are then be freeze-thaw cycled from one to anynumber of times to produce varying densities of physical associations,depending on the number of cycles. The cryogels are kept in the castsand then irradiated with 1-1000 kGy. The gels are then immersed in DI at80° C. to remove the physical associations.

As noted above, pores increase in size with the number offreezing-thawing cycles. It is thought that the polyvinyl polymer isrejected from the ice crystals as an impurity and is progressively“volume excluded” into increasingly polyvinyl polymer rich areas. Asmight be expected, the pore size increases with decreasing concentrationof polyvinyl polymer.

Gradients in pore size are produced as follows. Aqueous solutions of PVAare made as described above and poured into thin film casts. The castsare freeze-thaw cycled 1 to 8 times to produce physical associations.The thawed cryogels are kept in the casts and then partially dehydrated(by any means). They can then be irradiated in a gradient pattern at 1to 1000 KGy. The gels are then immersed in DI at 80° C. to remove thephysical associations. Regions that are shielded from irradiation willhave low to no junction points once the material is raised above itsmelting point, and will therefore have a pore left behind in theseregions. Depending on the size of the gradient pattern, nanometer tomillimeter sized holes can be made.

The claims should not be read as limited to the described order orelements unless stated to that effect. Therefore, all embodiments thatcome within the scope and spirit of the following claims and equivalentsthereto are claimed as the invention.

1. A method of making a covalently cross-linked vinyl polymer hydrogelcomprising the steps of: providing a physically associated vinyl polymerhydrogel having a crystalline phase; exposing the physically associatedvinyl polymer hydrogel to an amount of ionizing radiation providing aradiation dose in the range of about 1-1,000 kGy effective to formcovalent crosslinks; and removing physical associations by exposing theirradiated vinyl polymer hydrogel to a temperature above the meltingpoint of the physically associated crystalline phase to produce acovalently cross-linked vinyl polymer hydrogel.
 2. The method of claim 1wherein the step of providing a physically associated vinyl polymerhydrogel having a crystalline phase includes the steps of providing avinyl polymer solution comprising a vinyl polymer dissolved in asolvent; heating the vinyl polymer solution to a temperature elevatedabove the melting point of the physical associations of the vinylpolymer; inducing gelation of the vinyl polymer solution; andcontrolling the gelation rate to form physical associations in the vinylpolymer hydrogel.
 3. The method of claim 1 wherein the vinyl polymer isselected from the group consisting of poly(vinyl alcohol), poly(vinylacetate), poly(vinyl butyral), poly(vinyl pyrrolidone) and a mixturethereof.
 4. The method of claim 1 wherein the vinyl polymer is highlyhydrolyzed poly(vinyl alcohol) of about 50 kg/mol to about 300 kg/molmolecular weight.
 5. The method of claim 1 wherein the vinyl polymer ishighly hydrolyzed poly(vinyl alcohol) of about 100 kg/mol molecularweight.
 6. The method of claim 1 wherein the vinyl polymer is apoly(vinyl alcohol) having a degree of hydrolysis of about 80-100percent.
 7. The method of claim 1 wherein the vinyl polymer is apoly(vinyl alcohol) having a degree of hydrolysis of about 95-99.8percent.
 8. The method of claim 1 wherein the vinyl polymer is apoly(vinyl alcohol) having a degree of polymerization of about 100 toabout 50,000.
 9. The method of claim 1 wherein the vinyl polymer is apoly(vinyl alcohol) having a degree of polymerization of about 1,000 toabout 20,000.
 10. The method of claim 2 wherein the vinyl polymersolution is about 0.5-50 weight percent solution of poly(vinyl alcohol)based on the weight of the solution.
 11. The method of claim 2 whereinthe vinyl polymer solution is about 1-15 weight percent solution ofpoly(vinyl alcohol) based on the weight of the solution.
 12. The methodof claim 2 wherein the vinyl polymer solution is about 10-20 weightpercent solution of poly(vinyl alcohol) based on the weight of thesolution.
 13. The method of claim 1 wherein the ionizing radiation isgamma radiation or beta particles.
 14. The method of claim 1 wherein theradiation dose is about 1-1,000 kGy.
 15. The method of claim 1 whereinthe radiation dose is about 50-1,000 kGy.
 16. The method of claim 1wherein the radiation dose is about 10-200 kGy.
 17. The method of claim1 wherein the radiation dose rate is about 0.1-25 kGy/min.
 18. Themethod of claim 1 wherein the radiation dose rate is about 1-10 kGy/min.19. The method of claim 1 wherein the radiation dose is within 20% ofthe optimum radiation dose.
 20. The method of claim 1 wherein theradiation dose is within 10% of the optimum radiation dose.
 21. Themethod of claim 1 wherein the radiation dose is within 7% of the optimumradiation dose.
 22. The method of claim 1 further comprising providingan irradiation mask.
 23. The method of claim 22 wherein the irradiationmask is a step mask.
 24. The method of claim 22 wherein the irradiationmask is a gradient mask.
 25. The method of claim 1 wherein the solventof the vinyl polymer solution is selected from the group consisting ofdeionized water, methanol, ethanol, dimethyl sulfoxide and a mixturethereof.
 26. The method of claim 1 wherein the irradiated vinyl polymerhydrogel is immersed in a solvent is selected from the group consistingof deionized water, methanol, ethanol, dimethyl sulfoxide and a mixturethereof while is exposed to a temperature above the melting point. 27.The method of claim 2 further comprising the step of subjecting thevinyl polymer solution to at least one freeze-thaw cycle.
 28. The methodof claim 2 further comprising the step of mixing the vinyl polymersolution with a gellant, wherein the resulting mixture has a higherFlory interaction parameter than the vinyl polymer solution.
 29. Themethod of claim 2 further comprising the step of dehydrating the vinylpolymer hydrogel.
 30. The method of claim 1 wherein substantially all ofthe physical associations are removed.
 31. The method of claim 1 whereinabout one percent to about ninety percent of the physical associationsare removed.
 32. A covalently crosslinked vinyl polymer hydrogelproduced by the method of claim
 1. 33. A method of making a covalentlycross-linked vinyl polymer hydrogel comprising the steps of: providing avinyl polymer solution comprising a vinyl polymer dissolved in asolvent; heating the vinyl polymer solution to a temperature elevatedabove the melting point of the physical associations of the vinylpolymer; inducing gelation of the vinyl polymer solution; controllingthe gelation rate to form crystalline physical associations in the vinylpolymer hydrogel; exposing the physically associated vinyl polymerhydrogel to a dose of ionizing radiation of about 1-1,000 kGy effectiveto produce covalent crosslinks; and melting the vinyl polymer hydrogelin a solvent to remove physical associations, thereby making acovalently cross-linked vinyl polymer hydrogel.
 34. The method of claim33 wherein substantially all of the physical associations are removedand the covalently cross-linked vinyl polymer hydrogel substantiallylacks physical associations.
 35. The method of claim 33 wherein aboutone percent to about ninety percent of the physical associations areremoved.
 36. A covalently crosslinked vinyl polymer hydrogel produced bythe method of claim
 33. 37. An article of manufacture comprising thecovalently crosslinked vinyl polymer hydrogel of claim
 36. 38. Thearticle of manufacture of claim 37 selected from a device for deliveryof active agents, a load bearing orthopedic implant, a bandage, atrans-epithelial drug delivery device, a sponge, an anti-adhesionmaterial, an artificial vitreous humor, a contact lens, a breastimplant, a stent and non-load-bearing artificial cartilage.